OVERLOAD AND HYPERPLASIA
muscle for increased skeletal muscle fiber number. All of the studies used nitric acid digestion and/or histologi- cal cross sections to assess changes in muscle fiber number.
Changes in skeletal muscle. Changes in muscle fiber number for individual studies are given in Table 2. Across all designs and categories, significant increases in muscle mass (90.50 6 86.50%, 95% confidence inter- val 5 61.59–119.34), fiber area (31.60 6 44.30%, 95% confidence interval 5 16.83–46.37), and fiber number
(15.00 6 19.60
%, 95 percent confidence interval 5
–46.37) were found (Fig. 1). Examination of out-
lier groups revealed no physiological reason to exclude them from the analysis. Increases in fiber area were approximately twice as great as increases in muscle fiber number (P 5 0.27). Changes in muscle mass, fiber area, and fiber number ranged from 6 to 318%, from 221 to 141%, and from 210 to 82%, respectively.
When partitioned according to fiber-counting tech- nique, larger increases in muscle fiber number were found by using the histological vs. nitric acid digestion method (histological 5 20.70%, nitric acid digestion 5 11.10%; Fig. 2). Changes in muscle fiber number catego- rized according to species examined are found in Fig. 3. Increases in fiber number were greater among those groups that used avian (20.95%) vs. mammalian (7.97%) species. Changes in muscle fiber number partitioned by type of overload are found in Fig. 4. Stretch overload (20.95%) yielded larger increases in muscle fiber num- ber than did exercise (11.59%) and compensatory hyper- trophy (5.44%). In addition, no statistically significant differences between changes in fiber number were found when data were partitioned according to type of control (intra-animal 5 15.20%, between animal 5 13.90%; P 5 0.82) or fiber arrangement of muscle (parallel 5 15.80%, pennate 5 11.60%; P 5 0.61).
This meta-analysis attempted to quantify the magni- tude of change in muscle (particularly muscle fiber number) as a result of mechanical overload. Across all designs and categories, mechanical overload resulted in increases in muscle mass, muscle fiber area (hyper- trophy), and muscle fiber number (hyperplasia). Not surprisingly, increases in fiber area were approxi- mately twice as great as increases in fiber number. It appears that hyperplasia in animals is greatest when certain types of mechanical overload, particularly stretch, are applied. The results of this investigation are similar to a recent narrative review that concluded that muscle fiber hyperplasia 1) consistently occurs as a result of chronic stretch, 2) rarely occurs with over- load in the form of compensatory hypertrophy, and 3) has produced mixed results when overload in the form of exercise is employed (8). Although it is well estab- lished that mechanical-overload training results in increased fiber area (hypertrophy), and thus increases in muscle mass, the contribution of increased fiber number (hyperplasia) to increases in muscle mass has been more controversial. However, there now exists quantitative evidence to support the fact that certain
types of overload, particularly stretch, result in in- creases in muscle fiber number. Unfortunately, it is beyond the scope of this investigation to examine the processes (satellite cell proliferation and longitudinal fiber splitting) responsible for such changes. The greater changes in muscle fiber number found in avian vs. mammalian species may not be the result of the species used so much as the fact that stretch was the mechani- cal overload employed on all avian species included in this meta-analysis. The fact that increases in fiber number were approximately twice as great when histo- logical vs. nitric acid digestion methods were used is consistent with previous investigations (5, 6). Because of the ability to directly count each fiber, the nitric acid digestion method is generally considered to be the more accurate method of assessing changes in fiber number. However, small fibers may be missed when this method is used (8).
Despite the knowledge that studies can be more objectively evaluated by using the meta-analytic vs. traditional narrative approach, potential limitations still exist. In general, the very nature of meta-analysis dictates that the meta-analysis itself inherits those limitations that exist in the literature. For example, a review article by Timson (29) led him to conclude that none of the animal models (stretch, exercise, or compen- satory hypertrophy) currently used to examine exercise- induced muscle enlargement truly represents the hu- man strength-training situation under all conditions. In addition, the fact that 11 of the 17 studies involved essentially the same authors could have resulted in biased results. In summary, the results of this study suggest that in several animal species certain forms of mechanical overload increase muscle fiber number.
The author thanks Dr. Russ Moore (Dept. of Kinesiology, Univer- sity of Colorado, Boulder, CO), Dr. Ben Timson (Dept. of Biomedical Science, Southwest Missouri State University, Springfield, MO), and Dr. Zung Vu Tran (College of Health and Human Sciences, University of Northern Colorado, Greeley, CO) for their assistance in the preparation of this manuscript.
Address for reprint requests: G. Kelley, Exercise Science, Dept. of Physical Education, Northern Illinois Univ., Anderson Hall, Rm. 233, DeKalb, IL 60115-2854.
Received 26 February 1996; accepted in final form 15 February 1996.
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